Chemical sweetening of fuel oil



United States Patent 3.839.955 CHEMICAL SWEETENING 0F FUEL OIL Elizabeth L. Fareri and John P. Pellegrini, Jr., Pittsburgh,

Pa, assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Filed Sept, E9, 1958, Ser. No. 761,941 6 Claims. (Cl. 208204) This invention relates to sweetening distillate fuel oils that contain objectionable amounts of rnercaptans and to the thus-sweetened oils. More particularl the invention relates to converting mercaptans in such oils to less objectionable sulfur compounds by contacting such oils with certain substituted olefinic compounds in the presence of a catalyst.

It has been found that distillate fuel oils containing substantial proportions of mercaptan sulfur tend to de posit copper mercaptide gels in the burner systems in which they are consumed. The mercaptide gel deposits are objectionable in the burner systems since they cause burner stoppages and other malfunctioning that requires individual correction. The copper mercaptide gels contain copper mercaptides, fuel oil and water and, when substantially free of fuel oil sludge, are yellow or amber gelatinous materials. Chemical analysis of a field gel residue obtained after separation of fuel oil and sludge from a gel removed from a burner installation suggests a copper alkyl mercaptide or basic copper alkyl mercaptide structure corresponding fairly closely to that of copper dodecyl mercaptide or basic copper dodecyl mercaptide.

Mercaptans, the compounds that form copper mercaptide gel, are found in distillate fuel oils in varying proportions, dependmg mostly upon the nature of the cmde oil from which the distillate fuel oil is derived. For example, dist llate fuel oils derived from some crude oils, e.g., Pennsylvania or some Southern Louisiana crude oils, are essentially mercaptan-free and do not normally tend to deposit copper mercaptide gel. On the other hand, raw or untreated distillate fuel oils derived from Middle Eastern crudes may contain intermediate proportions of mercaptan sulfur, e.g., up to 0.01 percent or more, and distillate fuel oils derived from still other crudes, such as West Texas crude oil, may contain mercaptan sulfur in very large proportions, e.g., 0.1 percent, or more. Unsweetened distillate fuel oils of these types will deposit mercaptide gels. Su'aight-run distillate fuel oils may contain mercaptans that boil in the dis late fuel oil range and that are present as such in the o. inal crude oil. in the case of catalytically cracked distillate fuel oils, the mercaptans in the oils may be those present in the original crude oil, but they may also comprise merca Jtans, including aromatic mercaptans, formed from high molecular weight sulfur compounds during the catalytic cracking reaction.

The copper from which copper mercaptide gel is formed may come partly from that present in the original crude oil in the form of organic complexes, but is considered to be mainly derived from copper that is solubilized by contact with mercaptan-containing fuel oils during storage in individual burner systems in which the oils are consumed. Solubilization of copper may be achieved by the action of acidic materials such as naphthenic acids upon copper or brass metal parts, and/ or by microbial action on'such copper or brass metal parts. Although mercapti e gel appears to form most readily on metallic brass parts such as the float controls, fuel iiter screens and the like, the mercaptide gel may be found anywhere in the burner system. Thus, gel which has formed on the brass float control of a given burner system may cause a stoppage in a fuel line at some distance from the float.

Although mercapt-ans have been known to be present in certain unsweetened distillate fuel oils for many years, the copper mercaptide gel deposit problem has only been encountered in relatively recent years and then only in relatively restricted geographic areas, This apparent anomaly can probably be explained by the fact that the non-gel-forming distillate fuel oils marketed in the past have for the most part contained very little or no mercaptan sulfur. In instances where raw distillate fuel oils having very high mercaptan sulfur contents were produced, such oils were customarily subjected to a sweetening treatment which reduced the mercaptan sulfur content to a level below that capable of forming mercaptide gel. The recent difficulties have apparently come about as a result of the greater use in recent years, particularly on the east coast, of distillate fuel oils derived from Middle Eastern type crudes. These oils con tain more or less intermediate proportions of mercapt-an sulfur that have evidently been sufficiently large to cause mercaptide gel deposits, although not necessarily sufficiently large to require sweetening from an odor or corrosion standpoint. The gel deposit problem can also be accentuated by the kind of mercaptans found in such oils.

The present invention relates to treating distillate fuel oils that contain objectionable proportions of mercaptan sulfur whereby such oils are rendered less objectionable from the standpoint of gel deposition, odor, corrosiveness, and the like, and whereby such oils are better adapted for use as fuels. We have found that fuels having such improved characteristics can be obtained by contacting a mercaptan-containing distillate fuel oil that normally tends to deposit mercaptide gel with at least a substantially equivalent proportion, based on the mercaptan content of the oil, of a substituted olefinic compound having an olefinic bond alpha-beta to a functional group, preferably a carbonyl group, that is capable of rendering the beta-carbon atom of the olefinic bond electrophilic, in the presence of a catalytic amount, preferably about 1 to 5 percent by weight of the substituted olefinic compound, of a strongly basic material. An example of a preferred substituted olefinic compound is Z-ethylbutyl acrylate, but other types of compounds can be used. Examples of preferred strong base catalysts are sodium methoxide and sodium ethoxide. Although we prefer to carry out the sweetening operation with substituted olefinic compounds and strong base catalysts that are miscible with the fuel oil in the proportions employed so that, no subsequent separation of treating agents is requir "this is not absolutely necessary, and we can carry 0 the process with substituted olefinic compounds and/or fuel oil-immiscible strong base catalysts that are essentially entirely immiscible with the fuel oil. In such cases the sweetened oil can be separated from the treating agents subsequent to the aforesaid contacting and prior to actual use. The invention is important in connection with distillate fuel oils that contain at least about 0.006 percent by weight of mercaptan sulfur, that is sulfur in the form of mercaptans.

Although the mechanism as such of the reactions involved in the present invention has not been exhaustively investigated by us in the laboratory, it appears that a functional group that is capable of rendering a betacarbon atom electrophilic in an olefinic linkage alphabeta to said functional group, that is, a functional group containing a member having electronegative properties, causes a partial displacement of an electron pair from the alpha-beta position in the direction of the functional group, whereby an activated form of the substituted olefinic compound is produced in which the beta-carbon atom is electrophilic. Thus, in the case of a substituted olefinic compound containing an olefinic linkage alphabeta to an elcctronegative functional group, such as a carbonyl group, the activated form will be produced in accordance with the following equation:

=l =o J( 1=r o A a 5 a The strongly basic catalyst presumably abstracts a proton from the hydrogen-sulfur bond in a mercaptan, thereby forming a thi'oalkyl fragment that contains an electron surplus. This thioalkyl fragment is therefore attracted to the electrophilic betacarbon atom of the activated substituted olefin. The catalyst, in turn, gives up its proton to the functional group, from which position the proton undergoes an electrorneric shift to the alphacarbon atom. These reactions proceed in accordance with the following equation:

i 'i o as H l l l l l oC=C-H o-oo=0 RS/B RS H Thus, the mercaptans are converted to relatively innocuous beta-thioalkyl derivatives of the substituted olefinic compound, whereby they are rendered less objectionable from the standpoint of mercaptide gel deposition, odor, and corrosivencss.

Normally, the sweetening reaction will proceed to completion spontaneously, with evolution of heat, at ambient atomspheric conditions in a very short time. However, in order to compensate for relativelysluggish reactivity in the case of some substituted olefinic compounds, it may be desirable to treat the mercaptan-containing fuel oil while it is still warm from some processing step involving heating the oil, 'such as distillation, and/ or to provide a storage period of reasonable length to permit the reaction to proceed to completion prior to marketing the treated fuel oil.

The use of a strongly basic catalyst is important in the present invention as a strongly basic material is required to induce the weakly acidic mercaptans to give up a proton. es the strongly basic material iunctions as a catalyst, rather than as a reactant, it need be employed in the reaction in small amounts only. Usually, a measurable effect will be obtained by as little as 0.001

'percent catalyst by weight of the substituted olefinic compound. A marked effect will ordinarily be obtained with the use of as much as 1 to- 5 percent by weight or the substituted olefinic compound, and accordingly catalytic proportions in this range are preferred. We prefer to employ catalysts that will be miscible with the fu l oil in the proportions employed, such as alkali metal alcoholates, for the reason indicated above. For example, there can be used sodium ethoxide, sodium metnoxide, potassium methoxide and potassium ethox1de. ilcoholic, e.g., n-butauolic, sodium and potassium hydroxide also can be used, as can organic bases such as trimethylbenzylammonium hydroxide and trioctylbenzylamrnomum butoxide. The presence in the sweetened distillate fuel oils of catalytic amounts of the strongly basic material are not detrimental to the oil, and in fact such materials can actually enhance the storage stability of the fuel oils. Although we prefer to employ oil-miscible catalysts, the use of such catalysts is 'not essential, and normally oil-immiscible catalysts such as aqueous or solid caustic soda, potassium hydroxide or strongly basic anionexchange resins of the quaternary ammonium hydroxide type can be used. Theterrns strong base and .strongly basic material are used herein in their normal sense to indicate bases which will produce a titration curve like that of a strong base such as sodium hydroxide.

Substituted olefinic compounds that contain an olefinic bond alpha-beta-to a functional group that is capable of rendering the beta-carbon atom of such bond electrophilic, i.e., that contains an electronegative substituent, can be used in accordance with the present invention. The substituted olefinic compounds that are useful for the purposes of this invention will, of course, contain at least 2 carbon atoms. Although such substituted olefins can contain any greater number of carbon atoms per molecule, in the interest of retaining oil-miscibility and of minimizing the proportion of the sweetening agent in the mercaptan-containing fuel oil, we prefer that the substituted olefin contain not more than about 22 carbon atoms per molecule. The substituted oletinic compounds, the use of which is included by this invention, need not contain only one unsaturated linkage and they also can contain other substituents, such as halogen, hydroxyl, or the like, that will not react preferentially with the mercaptan or the catalyst. Especially good results are obtainable with substituted olefinic compounds that contain an olefinic linkage alpha-beta to a carbonyl functional group, a preferred member of the class being Z-ethylbutyl acrylate. An example of another material of the preferred class is Oxo-octyl acrylate.

Although we prefer to treat the mercaptan-containing distillate fuel oils with a substituted olefinic compound the class disclosed herein, that will be miscible with the oil in the proportions employed, so that the treating agent may be allowed to remain in the oil and so that no separation step is necessary, this is not absolutely essential, and we can carry out the sweetening process using a substituted olefinic compound, either liquid or solid, that is completely oil-immiscible. In such modifications the spent or partially spent treating agent can be separated from the fuel oil after the mercaptans have been converted to thioalkyl deriavtives of the substituted olefinic compound. In these modifications, the spent or partially spent treating agent can be regenerated after separation from the oil by treatment with lead acetate and a base. In such modifications the strongly basic catalyst can be either immiscible or miscible with the oil in the proportions employed. In accordance with one embodiment involving an entirely oil-immiscible treating agent and catalyst, the substituted olefinic compound and the catalyst are combined in one oil-immiscible macromolecule, an example of which is the hydroxide form of a quaternized, chloromethylated copolymer of styrene and about 12 percent divinylbenzene, that has been quaternized with 3 -dimethylamino-l-cyanopropene-1. The present invention also contemplates the use of oilmiscible treating agents in Whichthe catalyst and the substituted olefinic compound are incorporated in one oil-miscible molecule. An example of such a material is (3-cyano-2-propenyl) trioctylammonium butoxide.

We prefer to employ the substituted olefinic sweetening agents in relatively small proportions. Normally, we prefer to employ the substituted olefinic compounds in at least substantially equivalent (1:1 equivalent ratio) proportions with the mercaptans contained in the distillate fuel oil. Although it is not absolutely necessary to employ the substituted olefinic compounds in proportions substantially in excess of an equivalent amount, we prefer to employ the substituted olefinic compounds in greater than equivalent proportions in order to facilitate relatively rapid and complete sweetening; For most 'rnercaptan-containing distillate fuel oils proportions in the range of about 0.01 percent to 0.1 percent by weight of the fuel oil will be sufficient to effect a substantial sweetening of the fuel oil. In some instances thesubstituted olefinic compound can be added in proportions as great as 0.2 percent by weight of the fuel oil. However, larger proportions can be used, although no further benefit is ordinarily obtained by the addition of more than 0.2 percent of the substituted olefinic compound, unless the mercaptan content of the fuel oil is proportionately large. In no instance should the sub stituted olefinic compound be added to the fuel oil in proportions such as to affect adversely the burning qualities, stability, or other performance characteristics of the fuel oil. The strongly basic catalyst will normally be employed in proportions in the range of about 0.0001 to 0.01 percent by weight of the fuel oil and preferably in the range of about 0.0002 to 0.0025 percent by weight of the oil, but other proportions can be used. The proportions discussed above apply equally in the case of substituted olefinic compounds and strongly basic catalysts, that are miscible or immiscible with the oil, but in instances where the catalyst and substituted olefinic compound are incorporated in the same molecule, the catalyst proportions can be the same as those for the sweetening agent.

The mercaptan-containing fuel oils can be contacted with the substituted olefinic compounds disclosed herein in any suitable way. In the case of treating agents that are miscible with fuel oil, the sweetening agents can be added as such directly to the fuel oil. To facilitate blending of the substituted olefinic compound and strongly basic catalyst with the fuel oil, the sweetening agent and catalyst may be incorporated in a mutual solvent such as benzene, toluene, kerosene, ethylene glycol monobtuyl ether, diethylene glycol monobutyl ether, or the like, prior to blending. In the case of entirely oil-immiscible substituted olefinic compounds and strongly basic catalysts, any conventional method of producing thorough intercontact of immiscible materials, such as countercurrent flow, percolation, agitation and settling, filtering or centrifugal separation, can be used.

The invention is further illustrated by the following specific example:

Example I A commercial, run of the refinery No. 2 fuel oil distillate consisting of a mixture of approximately percent catalytically cracked fuel oil distillate and 90 percent straight run fuel oil distillate, of a kind found to involve a mercaptide gel deposit problem in domestic heating service, is sweetened by incorporation therein of 0.09 weight percent Oxo-octyl arcylate and 0.0045 weight percent sodium methoxide. The Oxo-octyl acrylate and sodium methoxide catalyst, being miscible with the fuel oil in the proportions indicated, are allowed to remain in the oil without separation. A typical sample of the oil referred to in this ex- Although the substituted olefinic compounds and catalyst mentioned in the preceding example constitute pre ferred materials in accordance with the present invention, it will be understood that the invention is not limited thereto. For example, good results will also be obtained by the use in the fuel oil of Example I in the same or equivalent proportions of 2-ethylbutyl acrylate.

The effectiveness of the herein disclosed combinations of substituted olefinic compounds and strongly basic catalysts to convert mercaptans to a less objectionable form has been stn'nkingly demonstrated by means of an accelerated test. In accordance with this test 1 gram of amyl mercaptan was dissolved in 20 milliliters of the fuel oil of Example I. To this solution there was added 1.6 grams of 2-ethylbutyl acrylate and 0.02 gram of sodium methoxide. Two and one-half hours later, to this mixture there was added 0.32 gram of copper naphthenate. No gel was formed. In the absence of Z-ethylbutyl acrylate and sodium methoxide, an otherwise identical test sample was completely congealed by the formation of copper amyl mercaptide.

No. 2 fuel oils are defined in ASTM Standards on Petroleum Products and Lubricants under the designation ASTM D396.

If desired, the distillate fuel oil compositions of this invention can contain in addition to the substituted olefinic compounds and catalysts previously described, other addition agents that improve the oils in one or more respects. For example, the oils can also contain oxidation inhibitors, corrosion inhibitors, combustion and/ or ignition improvement agents, sludge inhibtors, and the like. Moreover, the fuel oils that have been sweetened in accordance with the present invention can be subjected to other treating processes, e.g., hydrogenation, caustic treatment, acid treatment, or the like, prior to contact with the erein disclosed treating agents.

The expression Oxo-octyl is used herein to designate mixed, highly branched isomeric octyl radicals, principally dimethylhexyl, derived from the corresponding mixed isomeric alcohols, produced by the well-known Oxo-synthesis process.

By oil-miscibility we mean a degree of oil-dispersibility sufiicient to form a stable suspension, whether a dispersion, an emulsion, a colloidal solution, or a true solution.

Obviously, other improvements and modifications of the invention as herein set forth may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed as are in dicated in the appended claims.

We claim:

1. The process of reducing the mercaptan content of a distillate fuel oil having an objectionable mercaptan content and normally tending to deposit mercaptide gel, comprising contacting said oil with a minor proportion of a substituted olefinic compound having an olefinic linkage alpha-beta to a functional group that is capable of rendering the beta-carbon atom of said olefinic linkage electrophilic, said compound being selected from the group consisting of alkyl esters of acrylic acid whose alkyl groups contain 6 to 8 carbon atoms, and a catalytic amount of a substantially non-aqueous strongly basic material, said minor proportion being at least equivalent to the mercaptan content of said oil, said catalytic amount being in the range of about 0.001 to 5 percent by weight of said minor proportion.

2. The process of claim 1 wherein said substituted olefinic compound and said strongly basic material are miscible with said oil in the proportions employed.

3. The process of claim 1 where said substituted olefinic compound and said strongly basic material are immiscible with said oil and wherein these materials ar subsequently separated from said oil.

4. The process of claim 1 where said small amount is in the range of about 0.01 to 0.2 percent by weight of said oil and said catalytic amount is in the range of about 0.0001 to 0.01 percent by weight of said oil.

5. The process of claim 1 wherein said substituted olefinic compound is 2-ethylbutyl acrylate.

6. The process of claim 1 wherein said substituted olefinic compound is Oxo-octyl acrylate.

References Cited in the file of this patent UNITED STATES PATENTS 2,068,850 Ellis Jan. 26, 1937 2,723,944 Chenicek Nov. 15, 1955 2,758,058 Schneider et al Aug. 7, 1956 2,893,952 Chenicek July 7, 1959 2,399,396 Adams et al. Aug. 11, 1959 OTHER REFERENCES Noller et al.: Chemistry of Organic Compounds, W. B. Saunders Co. (copyright 1951, reprinted 1956), pp. 723-726 (only page 724 relied on). 

1. THE PROCESS OF REDUCING THE MERCAPTAN CONTENT OF A DISTILLATE FUEL HAVING AN OBJECTIONABLE MERCAPTAN CONTENT AND NORMALLY TENDING TO DEPOSIT MERCAPTIDE GEL, COMPRISING CONTACTING SAID OIL WITH A MINOR PROPORTION OF A SUBSTITUTED OLEFINIC COMPOUND HAVING AN OLEFINIC LINKAGE ALPHA-BETA TO A FUNCTIONAL GROUP THAT IS CAPABLE OF RENDERING THE BETA-CARBON ATOM OF SAID OLEFINIC LINKAGE ELECTROPHILIC, SAID COMPOUND BEING SELECTED FROM THE GROUP CONSISTING OF ALKYL ESTERS OF ACRYLIC ACID WHOSE ALKYL GROUPS CONTAIN 6 TO 8 CARBON ATOMS, AND A CATALYTIC AMOUNT OF A SUBSTANTIALLY NON-AQUEOUS STRONGLY BASIC MATERIAL, SAID MINOR PROPORTION BEING AT LEAST EQUIVALENT TO THE MERCAPTAN CONTENT OF SAID OIL, SAID CATALYTIC AMOUNT BEING IN THE RANGE OF ABOUT 0.001 TO 5 PERCENT BY WEIGHT OF SAID MINOR PROPORTION. 